1. Field of the Invention
The present invention relates to a charge pump circuit having a novel structure, more specifically, a semiconductor device using a charge pump circuit as a step-up circuit or a step-down circuit.
2. Description of the Related Art
A step-up circuit includes that using a coil and that using a capacitor. The one using a capacitor is generally called a charge pump. A conventional charge pump has a structure in which two diodes are connected in series and has a problem of a drop in output voltage by the same amount as a threshold voltage of the diode. In order to solve this problem, another structure using a switch instead of a diode has been proposed (see References 1: Japanese Published Patent Application No. 2001-136733, Reference 2: Japanese Published Patent Application No. H07-327357, and Reference 3: Japanese Published Patent Application No. H07-099772).
In order to realize the circuit disclosed in Reference 1, a circuit for stepping up a voltage to be equal to or higher than an external power supply voltage is needed to turn on or off a switch including a transistor (hereinafter also referred to as a TFT).
In general, a transistor is turned on by inputting Low potential to a gate terminal when the transistor is a p-channel transistor. This Low potential is lower than the potential of a source terminal of the p-channel transistor, and a potential difference between the Low potential and the potential of the source terminal of the p-channel transistor is equal to or lower than a threshold voltage of the p-channel transistor. Further, an n-channel transistor is turned on by inputting High potential. This High potential is higher than the potential of a source terminal of the n-channel transistor, and a potential difference between the High potential and the potential of the source terminal of the n-channel transistor is equal to or higher than a threshold voltage of the n-channel transistor. Note that the threshold voltage of a normal p-channel transistor is lower than 0 V. Further, the threshold voltage of a normal n-channel transistor is higher than 0 V. Therefore, when a gate-source voltage of a transistor is 0 V, the transistor is turned off and a current does not flow. Such a transistor is referred to as an enhancement mode transistor (also referred to as a normally-off transistor).
On the other hand, there is a transistor in which a current flows even when a gate-source voltage thereof is 0 V. Note that such a transistor is referred to as a depletion mode transistor (also referred to as a normally-on transistor).
In general, a transistor is manufactured to be normally off. In a charge pump included in a display device, if a switch including a transistor is normally off, an operating voltage of the transistor is increased. Therefore, the output of a step-up circuit from outside also needs to be increased. Accordingly, a circuit scale is increased, which leads to an increase in circuit area, a decrease in yield, and an increase in power consumption.
In addition, a conventional charge pump switching element as described above has the following problem when incorporated in a display device. A normal charge pump does not function to feed back an output voltage to stabilize output like other switching regulators. Therefore, the conventional charge pump switching element has a problem of loss of power source stability when a current load is heavy and an output current is increased.